How has "if it weren't for" got the meaning of
use this when you would do something different if a particular situation did not exist now
(the definition of "if it weren't for" in LDOCE) ?
EDIT 18 NOV
If we take "it" as "the situation" and "for" as "because of" as suggested by athlonusm, "if it weren't for XXX" will mean "if the reason for it isn't XXX even though I know it is in reality". Then for example
If it weren't for music, world would be a dull place.
If the reason for it isn't music even though I know it is in reality, world would be a dull place.
It sounds illogical, doesn't it?
Since the answers posted so far are explaining mainly what the subjunctive mood is, I have to emphasize that I know the basic meaning and usage of subjunctive "were".
I wanted to show using the above example about music that its current meaning is beyond its literal meaning. I suppose that there is a historic reason for it to have the current meaning. I want to know that.